1. Field of the Invention
The present invention relates to an optical mixer without disturbance of optical paths, notably for helmet visual display systems.
2. Description of the Prior Art
Helmet display systems are generally used in military aircraft or helicopters. Among the different types of display systems of this type, there are those fitted out with two image sources: a cathode-ray tube and a light intensifier tube (for night vision). The cathode-ray tube is used by day and by night to present a synthetic image (comprising piloting, navigation and firing information). The image intensifier tube is used only by night to enable a night vision of the outside scene without illumination. The images from the two sources are presented to the pilot's eyes by means of an optical collimation system common to both channels. This calls for a mixer in the optical path in order to enable the superimposition of the images of these two sources. This mixer is generally a semi-reflective mirror.
To work properly, this display system must meet the following constraints as regards the proportions of light reflected and light transmitted by the mixer. The light intensifier tube gives a luminance of some Cd/m.sup.2. The luminance of the image given by the light intensifier and considered at the pilot's eyes should also be equal to some Cd/m.sup.2. The latter luminance is directly related to the coefficient of refection of the mixer. This coefficient should therefore be the maximum. The cathode-ray tube may give a luminance of several thousands of Cd/m.sup.2. By night, the image of this cathode-ray tube should have a luminance of some Cd/m.sup.2. The coefficient of transmission of the mixer may therefore be very low in this case. Thus, by night, the typical characteristics of the mixer are: reflection : 90% for the light intensifier, and transmission: 10% for the cathode-ray tube. By day, the image of the cathode-ray tube should have a luminance of 1000 to 2000 Cd/m.sup.2 to be visible on a very bright background. Consequently, the mixer defined for the night is no longer suited to daytime use for it excessively attenuates the light of the cathode-ray tube.
Because the spectral bands of the images emitted by the cathode-ray tubes are very close to those of the images emitted by the light intensifier tubes, it is not possible to resort to standard selective mixers, which are not sufficiently selective.
One approach would consist in removing the mixer during daytime use since the light intensifier channel is then not used by day. The space taken up by the standard moving mechanisms and weights are non-negligible and incompatible with helmet visual display systems. Furthermore, the elimination of the mixer from the optical path would lead to a variation of the optical path. This would change the position of the images presented to the user and make it necessary to carry out an adjustment of this optical path or would lead to a lowering of the quality of the image.
Furthermore, since the mixer is made of a material having a refraction index different from 1, it has an undesirable optical influence: a transversal shift of the image, difference in focusing and other optical aberrations.
It is possible to consider replacing the mixer by an element made of glass (or of another optical material) with the same dimensions, without any coating for daytime use, in order to transmit the image of the cathode-ray tube without any attenuation and without contributing any optical faults. However, the implementation of such an approach would, in most cases, be complicated and lengthy and would require mechanisms that are bulky and difficult to control.